add tl;dr to issue templates

.github/ISSUE_TEMPLATE/bug_report.md

@@ -4,7 +4,13 @@ about: Create a report to help us improve
 labels: bug
 ---
 
-<!-- Please search open and closed issues to avoid duplicates. Maybe what you want has already been discussed or is mentioned in the readme.
+<!--
+tl;dr
+* search for duplicates also in closed issues
+* one issue per topic
+* reduce screenshot size
+
+Please search open and closed issues to avoid duplicates. Maybe what you want has already been discussed or is mentioned in the readme.
 If you want to report more issues / bugs, open multiple issues (one for each).
 If you're interested, you can read the following useful text about effective bug reporting (a bit longer read): https://www.chiark.greenend.org.uk/~sgtatham/bugs.html -->
 

.github/ISSUE_TEMPLATE/feature_request.md

@@ -5,6 +5,11 @@ labels: enhancement
 ---
 
 <!--
+tl;dr
+* search for duplicates also in closed issues
+* one issue per topic
+* reduce screenshot size
+
 Please search open and closed issues to avoid duplicates. Maybe what you want has already been discussed or is mentioned in the readme.
 If you have more than one feature request, open multiple issues (one for each) unless the requests are closely related.
 Note that given the number of feature requests and available developer time, it may take a long time until the feature request gets attention.

.github/ISSUE_TEMPLATE/other.md

@@ -4,6 +4,11 @@ about: Anything that does not fit into the other categories. Please don't use th
 ---
 
 <!--
+tl;dr
+* search for duplicates also in closed issues
+* one issue per topic
+* reduce screenshot size
+
 Please search open and closed issues to avoid duplicates. Maybe what you want has already been discussed or is mentioned in the readme.
 Open one issue per topic / request, and do not use this category for bug reports or feature requests.